1. Field of the Invention
This invention relates to a sound signal processing device comprising a frequency characteristic adding means for adding a desired frequency characteristic to an input sound signal and an automatic level control means for automatically controlling the signal amplification factor to prevent the output level from exceeding an allowable range. More particularly, the present invention relates to a sound signal processing device and a sound signal processing method for controlling the signal level according to the predetermined value of the frequency characteristic adding means and the value of the level conversion means of the system it belongs so as to secure a maximal dynamic range.
2. Related Background Art
Sound signal processing devices comprising an equalizer that is a frequency characteristic adding means for adding a desired frequency characteristic to a sound signal and an automatic level control means for automatically controlling the signal amplification factor so as to prevent the output level from exceeding an allowable range have been used in sound reproducing systems.
Particularly, the applicant of the present invention discloses a technology relating to an automatic level control means in Japanese Patent Application Laid-Open Publication No. 9-93063. The above patent document describes an automatic gain control circuit for equalizing the level of the input signal or limiting the level of the input signal to make its level from rising excessively.
FIG. 1 of the accompanying drawings is a schematic circuit diagram of a sound signal processing device 50 using an automatic level control means as disclosed in the above patent document. Referring to FIG. 1, the known sound signal processing device 50 comprises an equalizer 52 adapted to add a desired frequency character to digital sound input signal Din that is input to it from input terminal 51 and a downstream automatic level control section 53. The sound signal processing device 50 applies a gain computed by gain computer 54 that is a component of the automatic level control section 53 to variable gain multilayer 55. Unless noted otherwise, the unit of dB (decibel) is used for all the numerical values and the signs that indicate the gain in this letter of specification. Thus, the sound signal processing device 50 can control the level of the output signal of the equalizer 52 and obtain digital sound output signal Dout of the system it belongs. A level converter 57 for converting the level of the digital sound input signal Din supplied to the input terminal 51 is arranged upstream relative to the equalizer 52. A level converter 58 for converting the level of the output signal of the automatic level control section 53 is arranged downstream relative to the automatic level control section 53. The system provides an effect of preventing the signal amplified by the equalizer 52 from going beyond an allowable range and being distorted when it becomes the output signal Dout.
FIG. 2 is a schematic circuit diagram of the gain computer 54 in the automatic level control section 53 of FIG. 1, showing the configuration thereof. The output signal to which a specific frequency characteristic is added by the equalizer 52 is input to the input terminal 59 and converted into a logarithmic value x by a log converting section 60 and then supplied to adder 61. A threshold value th to which a negative sign is added is also supplied from threshold holding section 62 to the adder 61. Thus, the adder 61 computes the difference between the output signal (value x) of the equalizer that is converted into a logarithmic value and the threshold value th. The threshold value th of this known device refers to that of the maximum output level that the automatic level control section 53 can output and corresponds to the threshold value th in FIG. 3 that illustrates the input/output characteristics of the automatic level control section 53. The difference computed by the adder 61 is then supplied to multiplier 63 that is adapted to multiply it by −1, which is a coefficient. The difference computed by the adder 61 is also used as the base of the switching control signal of switch 65. The multilayer 63 multiplies the difference by −1 and supplies the product to anti-log converting section 64. The anti-log converting section 64 reduces the value x that is multiplied by −1 to an anti-logarithmic value and supplies the obtained value to terminal to be selected 65a of the switch 65. The switch 65 has terminal to be selected 65b in addition to the terminal to be selected 65a. The terminal to be selected 65b is connected to coefficient holding section 66 that holds coefficient 1. Thus, as the switch 65 turns movable piece 65c either to the terminal to be selected 65a or the terminal to be selected 65b according to the switching control signal that is based on the difference, either the anti-logarithmic value or the coefficient 1 is selected as the output of the switch 65. As pointed out above, the difference that is the difference (x−th) of the output signal (value x) of the equalizer and the threshold value (th) is output from the adder 61 when the difference (x−th) exceeds 0 ((x−th)>0) and hence the level of the value x is higher than the threshold value th, the difference operates as switching control signal for connecting the movable piece 65c of the switch 65 to the terminal to be selected 65a so that consequently the movable piece 65c is connected to the terminal to be selected 65a. As a result, the gain computer 54 supplies a value the suppresses the difference between the output signal of the equalizer and the threshold value th to variable gain multilayer 55 in FIG. 1. When, on the other hand, the difference (x−th) is not higher than 0 ((x−th)≦0) and hence the level of the value x is not higher than the threshold value th, the difference operates as switch control signal for connecting the movable piece 65c of the switch 65 to the terminal to be selected 65b so that consequently the movable piece 65c is connected to the terminal to be selected 65b. As a result, the gain computer 54 gives a one fold gain to the variable gain multilayer 55 in FIG. 1.
Assume that a flat sound input signal of 0 dB is input to the sound signal processing device 50 having the above described configuration and the equalizer 52 adds a frequency characteristic having a gain of G as peak value as shown in FIG. 4(1) to the signal. Note that FIG. 4 shows spectral graphs at a given instant. In the graphs of FIG. 4, the horizontal axis represents the frequency and the vertical axis represents the level. Assume also that the value of the gain of the level converter 57 of FIG. 1 is −A, whereas that of the gain of the level converter 58 is A (G≦A) and these values are fixed. Then, the threshold value th of FIG. 2 needs to be equal to −A in order to prevent the output signal Dout of FIG. 1 from exceeding 0 dB and being distorted if gain A is added by the level converter 58.
(1), (2) and (3) in FIG. 4 respectively illustrate the outputs at (1), (2) and (3) in FIG. 1 when a flat signal of 0 dB is input to the system. Firstly, the flat input signal of 0 dB is attenuated by A by the level converter 57 of FIG. 1 and subsequently a frequency characteristic having a gain with a peak value of G is added by the equalizer 52. Thereafter, the automatic level control section 53 shown in FIG. 1 operates for level control in such a way that output signal 56 does not exceed 0 dB and hence is not distorted if gain A is added thereto by the level converter 58. At this time, since peak level of the output of the equalizer 52 is (−A+G) and the threshold value th of FIG. 2 is −A, the value given from the gain computer 54 to the variable gain multilayer 55 in the automatic level control section 53 is equal to ((−A+G−(−A))×(−1))=−G due to the arrangement of FIG. 2. Thus, the signal level is attenuated by G from (1) so as to become (2) in FIG. 4. Then, ultimately, the signal is amplified by A as shown i (3) of FIG. 4 by the level converter 58 before it is output. The gain −A of the level converter 57, the gain A of the level converter 58 and the gain −A that is the threshold value th of FIG. 2 are constant and never vary regardless of the characteristic that is added by the equalizer 52. Therefore, if G<A, it will be understood that a dynamic range loss of (A−G) arises in the signal processing operation as shown in (1) of FIG. 4.
Assume here that a volume is added to the sound signal processing device 50 of FIG. 1 so as to control the sound volume of the entire system. The volume 59 is arranged most downstream in the system of FIG. 1 such that it can be controlled independently. Since the volume 59 is independent of the equalizer and the automatic level control section, if the level control operation is conducted or not depends solely on the input level of the signal. If the gain value of the volume 59 is −A and 0 dB is input to the system, the output at (1) and the one at (2) in FIG. 5 are entirely same as those shown in FIG. 4 (as shown in (1) and (2) in FIG. 6) and the output at (3) in FIG. 5 is same as the one shown in (3) of FIG. 6 because it is obtained by shifting (3) in FIG. 4 by the gain value −A of the volume 59 in FIG. 5. In this case, if the level converter 58 is variable and it is known that the level is raised by A at the level converter 58 and then lowered by −A at the volume 9, 0 dB can be selected for the gain value of the level converter 58 and that of the volume 59. If 0 dB is selected for the gain value of the level converter 58 and that of the volume 59, the output signal Dout would not exceed 0 dB and become distorted. Therefore, it is not necessary to conduct a level control operation as shown in (2) of FIG. 6 by operating the level control section 53. However, a level control operation that is supposed to be unnecessary as described above may have to be conducted since the gain −A of the level converter 57, the gain A of the level converter 58 and the threshold value th of FIG. 2 are always fixed.
Thus, a known sound signal processing device comprising an equalizer and an automatic level control section as principal components thereof is accompanied by a problem that a dynamic range loss may arise as pointed out earlier and a problem that a level control operation that is supposed to be unnecessary may have to be conducted. These problems give rise to degradation of the sound quality.